I've never done a tri, but have done 1 du and am planning on 3 this year (one is not a "true" duathlon, as it only has a single run leg).

Transitions are the same, I think. Instead of a first swimming leg you'll have a run leg, then bike, then back to run. We have one organization that manages the majority of duathlons around here...I've also found some on runningintheusa.com and by word-of-mouth. You might find something on duathlon.com, too. I can't recall what was listed there for my area when I checked, but I'm pretty sure they have an event calendar of some sort.